I am beginning to doubt that it is the FTDI drivers. The green LED is tied to the 485 chip and if either it or the opto fails, it will not light. I would check the board and make sure that there are no cold solder joints. If you have an LE, you could borrow one of its 485's and see if that is the issue. If you reflash the PIC, make sure that all of the pins are seated properly in the socket.